1. Field of the Invention
The present invention relates to an apparatus for measuring the properties of acoustical transducers and, more particularly, to an apparatus for measuring sensitivity and droop in a condenser microphone in such a manner that a more accurate measurement of the sensitivity and low frequency cutoff of the microphone can be made than heretofore was possible.
The peak amplitude and subsequent droop constitute the electrical response of a microphone to a step function input. When a positive or negative step function input is applied to a microphone in the form of an sudden or instantaneous increase or decrease in air pressure, the microphone voltage output will show the leading edge of the step function. However, the voltage output will decay from the instantaneous peak level back to the original zero level. This rate of this decay is defined as droop and may be used to determine the low cutoff frequency of the microphone. The instantaneous peak voltage level determines the microphone sensitivity.
The cause of this decay is found in the construction of the microphone. Condenser microphones are constructed with a movable plate and a fixed plate parallel to each other surrounded by a housing thereby defining a cavity between the parallel plates. The movable plate is subject to physical deformation in response to changes or differentials in air pressure between the cavity and the outside face of the movable plate. A small orifice or vent located in the housing equalizes the air pressure within the cavity with that exerted on the microphone outside the cavity.
When a positive or negative step function is applied to the microphone, the movable plate is displaced resulting in a voltage output from the microphone. The orifice or vent described above equalizes the air pressure in the cavity with the air pressure exerted on the microphone from outside the cavity. As this happens, the movable plate returns to its original position at a rate determined by the orifice size and the volume of the cavity. Of course, as the movable plate return to its original position the voltage output of the microphone decays to zero or a reference level. This rate of decay is the droop of the microphone.
2. Description of the Prior Art
Devices heretofore employed for the purpose of measuring droop in condenser microphone have suffered from the mechanical limitation introduced by the utilization of a positive step function input for testing and measuring purpose.
In particular, to apply a positive step function input to a condenser microphone, the microphone must be placed in an rigid, airtight enclosure or housing and the pressure within the enclosure quickly increased and maintained at a constant level. This requires a mechanical system capable of increasing the pressure quickly enough to effect the fast rise time that is required to minimize pressure equalization via the microphone housing orifice while the positive step function is being applied. Notwithstanding the fact that such prior art mechanical systems are complex and expensive to build they additionally suffer from the slow response limitations inherent in all mechanical systems which prevents them from achieving an acceptably fast rise time.
In addition, when a condenser microphone is placed in a rigid enclosure, the microphone is subject to reflections or acoustic noise resulting from pressure waves bouncing off the surfaces of the enclosure surrounding the microphone. These reflections or noise signals result in distortions or noise signals appearing in the electrical response of the microphone to the positive step input thereby introducing inaccuracies into the droop measurement as well as the sensitivity measurement.
Also an accurate method of controlling and measuring the step pressure level is necessary when making droop and sensitivity measurements. Prior art devices are unable to effect accurate measurement and control of the step pressure level at the low pressures needed to test the microphone.